This invention relates generally to the use of machine vision systems for semiconductor chip bonding/attaching devices. More specifically, the present invention relates to the use of a corner cube retro-reflector as an offset alignment tool that acquires indirect images of the bonding target during the die attach process when the same lie outside the view of the imaging system. From such images, coordinate information on position can be obtained and any positional offset from a reference position of the die bonding tool or die placement tool due to deviations caused by thermal change or other nonrandom systemic errors can be taken into account for correct alignment of wire bonding, die placement or fiber placement tools.
Semiconductor devices, such as integrated circuit chips, are electrically connected to the leads on a lead frame by a process known as wire bonding. The wire bonding operation involves placing and connecting a wire to electrically connect a pad (first bond) residing on the semiconductor die to a lead (second bond) in a lead frame. Both the first and the second bonds have to be placed accurately as dictated by requirements of the package. Once all the appropriate pads on the chip have been wire bonded to the appropriate leads on the lead frame, it can be packaged, often in ceramic or plastic, to form an integrated circuit device. In a typical application, a die or chip may have hundreds or thousands of pads and leads that need to be connected.
There are many types of wire bonding equipment. Some use thermal bonding, some use ultra-sonic bonding and some use a combination of both. Prior to bonding, vision systems or image processing systems (systems that capture images, digitize them and use a computer to perform image analysis) are used on wire bonding machines to align devices and guide the machine for correct bonding placement.
In conventional systems, post attach inspection is used to determine if relative changes in bonding or die placement tool position are necessary to effect proper and accurate placement of die or wire bonds. As such, these conventional systems can only compensate for improper wire bonds or improper die placement after such improper wire bonds or improper die placement actions occur, thereby negatively effecting device yield and machine throughput. These conventional systems have additional drawbacks in that they are unable to easily compensate for variations in the system due to thermal changes, for example. These changes require periodic inspection of completed devices further impacting device yield and negatively impacting manufacturing time.
In conventional systems the vision system (shown in FIG. 11) consists of two image devices, a first image device 1104 placed below the optical plane 1112 and upwardly viewing objects and a second image device 1102 placed above the optical plane and downwardly viewing objects. These conventional systems have drawbacks in that in addition to requiring more than one image device, they are unable to easily compensate for variations in the system due to thermal changes, for example.
In view of the shortcomings of the prior art, it is an object of the present invention to provide a method for attaching an integrated circuit die to an underlying substrate using vision system that takes into account variations due to temperature changes and other nonrandom systemic effects.
The present invention is a vision system for use with a semiconductor fabrication machine for accurate die alignment and die placement. The system comprises an alignment tool having a plurality of internal reflection surfaces, the alignment tool located below a vision plane of the substrate; and an optical detector to receive an indirect image of a bottom surface of the die through the alignment tool.
According to another aspect of the invention, the vertex of the alignment tool is located at a position about midway between an optical axis of the optical detector and an optical axis of the die.
According to a further aspect of the invention, the alignment tool comprises a plurality of cornercube offset tools.
According to still another aspect of the invention, the focal plane of the vision system is positioned at or above the alignment tool.
According to yet another aspect of the present invention, the system includes a lens positioned between the alignment tool, and i) the optical detector and ii) the die.
According to still another aspect of the present invention, the system includes a first lens positioned between the optical detector and the alignment tool and a second lens positioned between the die and the alignment tool.
According to a further aspect of the present invention, the first lens and the second lens are located at or below the image plane.
According to another aspect of the present invention, the first lens and the second lens are located in line with the image plane.
According to yet a further aspect of the present invention, the reflecting surfaces are three mutually perpendicular faces.
According to yet another aspect of the present invention, the angle between each of the internal reflective surfaces and the top surface of the corner cube offset tool is about 45xc2x0.
According to still another aspect of the invention, the optical detector is a CCD camera.
According to yet another aspect of the invention, the optical detector is a CMOS imager.
According to yet a further aspect of the invention, the optical detector is a position sensitive detector.
According to an exemplary method of the present invention, a cornercube offset tool is positioned below a vision plane of the die; a lens is positioned between i) the die and the cornercube offset tool and ii) between an optical imager and the cornercube offset tool; and the die is viewed indirectly through the cornercube offset tool and the lens.
These and other aspects of the invention are set forth below with reference to the drawings and the description of exemplary embodiments of the invention.